Ink jet printers operate by ejecting tiny drops of ink from a printhead onto a printing medium, such as paper. The printhead normally includes a nozzle plate having a plurality of nozzles through which tiny ink droplets are ejected onto the paper to collectively create an image. To deliver ink to the nozzles, the printhead includes a plurality of ink firing chambers, each fluidically connected to an associated nozzle through a bore. Within each firing chamber is a heat-generating resistor that is selectively excited to heat the ink in the chamber, which creates a bubble. As the bubble expands, some of the ink is forced through the bore out of the nozzle onto the paper. A plurality of ink drops collectively form a desired image on the paper.
The quality of the resulting image depends in part on the trajectory of the ink drops as they are ejected from the printhead nozzles. Poor ink drop trajectory and velocity are sometimes caused by ink puddles that form at the nozzle exit. In some cases, ink puddles are the result of poor control over the ink drop as the ink enters the bore and is ejected from the nozzle. In other cases, ink puddles are the result of ink overshooting, ink drop breaks, and hydrophilic (water attracting) nozzle surfaces. Excessive ink puddling can not only distort the trajectory of the ink drop, but it can also cause intermittent nozzle shutdown preventing any ink from ejecting onto the paper therefrom.
Prior attempts to prevent ink from puddling at the nozzle exit include using ink formulations that incorporate additives to inhibit puddling. Unfortunately, such additives can negatively affect the ink and are not chemically compatible with all printing systems and can cause damage to some internal components of the printhead.
Another previously attempted solution includes applying a non-wetting, hydrophobic coating to the outer surface of the nozzle plate to inhibit the ink from adhering to the outer surface of the nozzle exit. However, providing a hydrophobic coating only to the exterior surface of the nozzle exit does not provide control over the position of the ink drop in the bore of the nozzle. As a result, excess ink remains in the bore after a drop has been ejected, causing additional puddling at the nozzle exit. The embodiments described hereinafter were developed in light of these and other drawbacks.